1. Field of the Invention
The present invention relates to a method of correcting lithographic masks used in the production of solid-state devices.
2. Description of the Prior Art
Lithographic shadow masks are used extensively in the production of solid-state devices, including semiconductor devices, magnetic bubble devices, etc. The pattern contained in the shadow mask is reproduced onto the surface of a device precursor, typically by the irradiation of a radiation-sensitive resist material on the precursor. The variety of radiation sources for lithography that have been used or proposed includes visible light, ultraviolet light, X-ray radiation, electrons, and ions. When illuminated by a broad beam of the radiation, the shadow mask serves to selectively block portions of the radiation beam while allowing other portions to be transmitted therethrough. In this manner, very complex geometries having very small minimum line widths can be reproduced, allowing the economical production of very large scale integrated circuits and other devices.
A single shadow mask is typically employed a large number of times for the production of numerous solid-state devices. In this manner, the precision and time-consuming techniques used to make the shadow mask need not be repeated for every device, but rather the precision inherent in the shadow mask is transferred to every device. However, this places stringent demands upon the quality of a shadow mask, since any flaws or defects are reproduced in the device precursor, which reduces the yield of usable devices. For typical projection printing, a mask having the same feature dimensions as the resulting device (1:1 printing) typically allows about 100 semiconductor chips to be simultaneously exposed to lithographic radiation on a single 4 or 5 inch diameter wafer at a given time. However, since the size of the feature on the mask is essentially the same as the resulting feature size in the device that is made, and since feature sizes are currently on the order of about 2 micrometers and constantly being reduced, it is apparent that the mask must be made with very high precision.
Another lithography technique makes use of a reduction lens, typically 5:1 or 10:1, which allows the features on the mask to be correspondingly larger than the features on the solid-state device. Thus, a 1 micron line can be reproduced with a 5 micron wide feature on a lithographic mask using a 5:1 reduction lens. However, such reduction systems typically expose only a single chip, or perhaps a small number of chips on a wafer, to the patterning irradiation at one time, rather than exposing an entire wafer at one time. Any defect on the mask that would cause the resulting device to malfunction results in essentially zero yield of usable devices, since all the devices are exposed through the same mask pattern. Hence, it is imperative that masks for the repeated exposure systems be essentially perfect; such systems are typically referred to as "steppers" in the art, as portions of a wafer are moved in steps under the shadow mask.
Combining the most troublesome features of both of the above techniques are the anticipated x-ray stepper lithography systems. In these systems, typically only one chip, or perhaps a few chips, are exposed at one lithography step on a given portion of a wafer. Hence, the mask must be substantially perfect, as noted above. However, no reduction lenses can be used, since X-rays are not conveniently refracted by available optical components. Hence, the 1:1 resulting reduction ratio implies that the features on the X-ray masks will be very small. It is anticipated that such masks will have feature sizes of 1 micron or less.
Optical masks (i.e., those for use with visible or ultraviolet light) are commonly repaired by laser evaporation of opaque chrome defects, and inking or lift-off chrome metallization for clear (pinhole) defects. Laser repair of opaque defects has a resolution typically limited to about 1 micron by the size of the focused laser spot and thermal diffusion due to the mask substrate. Precise control of feature size and placement is not easily achieved with laser evaporation. This can make edge reconstruction of fine features difficult, particularly in tight geometries, that is, those having minimum dimensions of about 2 micrometers or less. The resolution and feature size and placement control for clear defect repair are generally worse, making edge reconstruction even more difficult.
Defects in X-ray masks tend to be opaque and at a density about 1 order of magnitude greater than optical masks. No attractive repair methods have been described for X-ray masks. Laser repair of these opaque defects does not seem feasible. Substantial thermal diffusion due to the large thickness of the metal absorber (typically about 0.5 to 1.0 micrometer gold for an X-ray mask versus about 800 Angstroms of chrome on an optical mask) results in poor resolution and control of feature size and shape. This is particularly troublesome because X-rays are likely to be used only for submicron lithography. Metal redeposition due to the high aspect ratio of the X-ray mask features, and damage to the supporting membrane, are also problems with prior art repair techniques.
In forming the mask initially (i.e., before repair), typically a metal coated substrate is coated with a radiation-sensitive resist and exposed to patterning radiation. Focused electron beams that scan across the resist and selectively expose the resist are frequently used as the patterning radiation. However, optical radiation is also used, especially with masks having relatively large features. The exposed resist is then developed and the pattern transferred into the metal layer by etching. It has been proposed to avoid the resist exposure and development steps entirely by directly patterning the metal layer with an ion beam that mills away the metal to form the pattern. However, that technique is entirely impractical with commercially useful masks for very large scale integrated circuits, using available ion sources. For example, at a beam current density of 1 amp/cm.sup.2 (e.g., a 1000 Angstrom diameter beam having a current of 10.sup.-10 amps), to pattern at 50 percent density an area of 1 cm.sup.2 in 850 Angstrom thick chrome, requires a time of over 4 years. Hence, workers in the art have not pursued the use of ion beams for direct formation (i.e., without resists) of complex patterns.